Radiation shield for an integrated circuit memory with redundant elements

ABSTRACT

A shield for an EPROM cell comprising an upper cover and upstanding elements. Generally MOS electrically programmable read-only memories (EPROMs), can be erased by exposure to radiation. Typically, the EPROM is encapsulated in a package which has a transparent lid to allow radiation to pass through the package to erase the EPROM cells. The invented shield protects selected EPROM cells from the radiation and thus prevent these cells from being erased even though the entire EPROM package is subjected to radiation and other EPROM cells are erased. The shielded EPROM cells are useful for discretionary connections such as those needed in redundant memories.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of MOS electrically programmableread-only storage devices which are capable of being erased by exposureto radiation.

2. Prior Art

Metal-oxide-semiconductors (MOS) electrically programmable read-onlymemories (EPROMs) have been utilized for the storage of information inthe prior art. A cell for such memories is described in U.S. Pat. No.3,996,657. These memories are erased by exposure to ultravioletradiation. Memory arrays which employ such cells are disclosed in U.S.Pat. Nos. 3,728,695 and 3,744,036.

Redundant rows and columns have been utilized in the prior art forreplacing defective rows and columns in read-only memories (ROMs) andrandom-access memories (RAMs). Fusible silicon links are used to enablethese redundant elements. (A process for making a PROM employing fusiblesilicon links is described in U.S. Pat. No. 3,792,319.) U.S. Pat. No.4,250,570 describes a memory employing redundant elements. Anotherredundancy system for memories is shown in Memory Redundancy Apparatusfor Single Chip Memories, Ser. No. 320,600 filed Nov. 21, 1981 andassigned to the assignee of the present invention.

It is an object of this invention to provide a shield for integratedcircuits with areas sensitive to radiation, some of which areas must beprotected from radiation.

It is another object of this invention to provide an improved redundantmemory circuit by shielding selected EPROM cells from radiation.

SUMMARY OF THE INVENTION

A shielding structure is provided for protecting certain radiationsensitive areas of an integrated circuit from radiation while permittingradiation to strike other radiation sensitive areas. Such shielded areasare particularly useful in a programming means for repairing defectiveEPROM arrays where the array is typically erased by exposure toultraviolet radiation. The invention is also useful to provide limitedcustomizing of EPROM products. For example, a shielded EPROM cell may beused to enable or disable certain features or may be used to program thelogic level required for a particular function. In the preferredembodiment, a plurality of EPROM cells are enclosed within a shieldingstructure comprised of a metal shield and associated cap such thatradiation entering the shielding structure is minimized and in any eventmust traverse a circuitous path before reaching the shielded EPROMcells.

The novel features which are believed to be characteristic of theinvention, is the use of shielded EPROM cells in conjunction with aprogramming means to permanently modify the functionality of an EPROM.The structure of the shield, together with further objects andadvantages thereof, will be better understood from the followingdescription in connection with the accompanying drawings in which thepresently preferred embodiment of the invention is illustrated by way ofexample. It is to be expressly understood, however, that the drawingsare for purposes of illustration and description only and are notintended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an EPROM which includes a redundant columnand a programming means which incorporates the teachings of the presentinvention to select such redundant column.

FIG. 2 is a pictorial view of the presently preferred embodiment of theinvention.

FIG. 3 is a topological layout of the presently preferred embodiment ofthe invention which uses a minimum of silicon substrate area.

FIG. 4 is a cross-sectional elevation view of the invention takenthrough section line 4--4 of FIG. 3.

FIG. 5 is a cross-sectional elevation view of the invention takenthrough section line 5--5 of FIG. 3. This view demonstrates the pathradiation will take after entering the shielding structure at anopening.

DETAILED DESCRIPTION OF THE INVENTION

A shield is described which is useful for protecting EPROM cells fromradiation so that electrically programmable read-only memory cells whichcannot be erased when exposed to radiation may be fabricated on the samesubstrate as electrically programmable read-only memory (EPROM) cellswhich can be erased when exposed to radiation. The term EPROM as used inthis application refers to a memory which can be electrically programmedand erased repeatedly by exposure to radiation. Erasure typically occurswhen the EPROM is exposed to specific doses of radiation. The inventionpermits selected EPROM cells to be effectively converted to programmableread-only memory (PROM) cells by shielding these selected cells fromradiation. A typical use of these shielded EPROM cells in an EPROM is ina programming means which disables defective elements and enablesequivalent (redundant) elements in the memory.

FIG. 1 is a block diagram of a memory with a redundant memory circuit.An array 10 is shown with a defective column 11. A programming means 12which includes shielded EPROM cells 13, is shown. Assume that a highstate on the output 14 selects redundant column 15. The cells 13 wouldbe programmed in such a manner that when defective row 11 is selected bythe address signals, the output of the programming means 14 will be in ahigh state. Consequently, redundant column 15 will be selected. When theoutput 14 is high, the output of inverter 16 will necessarily be low.The output of inverter 16 is connected to the input of all of the ANDgats 17 which are used to decode the column address. Since the output ofinverter 16 is connected to the input of each AND gate 17, the output ofeach AND gate 17 must necessarily be low when the output of inverter 16is low. Hence, when defective column 11 is addressed, column 11 isdisabled and the redundant column 15 is selected instead. The cellswhich make up array 10 are EPROM cells which are erased when exposed toradiation. The cells 13 in the programming means may be the same EPROMcell but such cells would be protected from radiation by the presentinvention. Thus, the cells 13 would be shielded EPROM cells and could beprogrammed once in such a manner that the redundant column 15 would beselected in place of the defective column 11.

The shielded EPROM cells may also be useful to customize an EPROM. Forexample, certain users of the EPROM may desire the EPROM to be selectedin a particular system when the chip select pin of the EPROM is in thehigh state. Other users would prefer that the EPROM be selected when thechip select pin is in a low state. The user could select the properlogic state by programming the shielded EPROM cell. The shielded EPROMcell would also be useful in providing the user with the option ofenabling or disabling a particular function. For example, suppose thattwo different functions were designed into an EPROM but that the numberof pins available required that both functions be connected to the samepin. Assume further that one function is useful to a certain class ofusers and the other function is useful to a second class of users. Theshielded EPROM cell would provide a means for the user to select thefunction of its choice. A further use of the shielded EPROM cell wouldbe to digitally adjust reference voltages (e.g., a particular referencevoltage would result depending on the number of shielded EPROM cellsprogrammed).

The prior art (such as U.S. Pat. No. 4,250,570) taught that fusiblesilicon links (fuses) could be used in a programming means to selectredundant elements. However, there are many problems associated withsuch fuses which do not exist when a shielded EPROM cell is used in theprogramming means in place of a fuse. The fuse is an additional circuitelement which must be characterized and monitored in addition to theother circuit elements normally used in an EPROM.

Another problem with the fuses is that they are difficult to manufacturein a consistent manner. Consequently, the fuses are not reliable. Somefuses may be blown at a particular current while other fuses require ahigher current to be blown. The unreliability of these fuses is asignificant problem associated with the prior art fuses. Furthermore,the layout of the silicon fuse requires a significant amount of siliconsubstrate area when compared to the EPROM cell layout. The larger layoutrequired for the fuse will result in a higher overall cost of the EPROM.Finally, an opening in the oxide is normally made above the fuse so thatthe fuse can be reliably blown and yet protected from blowingspuriously. Such an opening permits foreign material to accumulate onthe fuse. This material causes reliability problems and is a significantproblem associated with prior art fuses.

All of the above problems are overcome by the use of a shielded EPROMcell in place of a fuse. No extra element is required since the shieldedEPROM cell is simply the standard EPROM cell used in the array which isprotected from radiation by a shielding structure. The shieldingstructure is fabricated as part of the deposition of an opaque materialsuch as metal (typically aluminum) in the standard EPROM process. Theshielded EPROM cell will be as reliable as the EPROM cell in the array.No openings in the oxide will be necessary since the cell is simply ashielded EPROM cell which is permanently programmed by the samemechanisms used to program the cells in the array.

The basic MOS device is a sandwich of many oxides which are transparentto radiation. It has been learned that while an opaque upper cover overthe cell prevents radiation from entering the cell perpendicular to theplane of the oxides, radiation still enters under the cover in the planeof the oxides. With just an upper cover there is nothing to stop theradiation from entering in the plane of the oxides from traveling to thecell area and thereby erasing the celUpstanding members extending downfrom the cover are needed to prevent radiation from entering in theplane of the oxides. These upstanding members would have to contact thesubstrate to prevent radiation from entering the gate oxide. However, abox-like shield made out of a conductor does not leave a path for otherconductors connected to the cells to communicate with other circuitry ofthe EPROM. Consequently, the cells could not be programmed nor onceprogrammed, could the state of the cell be sensed.

A box-like shield without an opening could be used as a shield if anopaque insulator (such as silicon nitride, paint, or photoresist) wasused to form the shield. The problem with this solution is thatadditional processing steps would be required to form the shield. Thesesteps would increase the cost of the process and require the performanceof significant reliability studies.

As discussed earlier, EPROMs will be repeatedly programmed and erased.During the erase cycle, they will typically be exposed to ultravioletradiation for approximately 30 minutes. The goal of the presentlypreferred embodiment is to prevent the floating gate from losingsignificant charge when exposed to radiation for 300 hours.

The invention is a shielding structure which minimizes the radiationthat can travel to the floating gate of the EPROM cell and particularlyminimizes the radiation that can enter the EPROM cell in the plane ofthe oxides. Polysilicon was used in experiments as a shielding structurefor the EPROM cells. Experimental data showed that a layout of oneshield made of polysilicon and one cap made of polysilicon impeded theradiation from reaching the shielded cell but the cells still lostsignificant charge when the EPROM was exposed to radiation.Consequently, a metal (aluminum) shield and metal cap were chosen forthe presently preferred embodiment of this invention.

FIG. 2 shows the shield 18 as a continuous structure comprised of afirst upper cover 48 and an upstanding member which includes sides 21,44, 49, and 50. The upstanding member extends down from the first uppercover to make contact with the substrate. The cap 19 is also acontinuous structure comprised of a second upper cover 48 and a secondupstanding member which includes sides 46, 51 and 52. The secondupstanding member extends down from the second upper cover 47 to makecontact with the substrate. Shield 18 and cap 19 form the shieldingstructure which protects the four EPROM cells 26-29 from radiation. Inan actual MOS device, the shield 18 and cap 19 will not have sharpcorners as shown in FIG. 2 but will be rounded during the actualfabrication process. It is important to note that the metal sides suchas side 21 extend down and make contact with the substrate. This isimportant to insure that radiation cannot enter through the gate oxide.

Light can only enter the shield through opening 22 or opening 23.Opening 23 defined by cap 19 provides a means for conductors to leavethe shielding structure so that the shielded EPROM cells can beconnected to other circuitry of the EPROM. This is necessary to programthe cells and to later sense the state of the cells. The presentlypreferred embodiment encloses four EPROM cells within a single shieldingstructure. The drain region of each EPROM cell, a common source regionand a common select gate must be available for connection to othercircuitry. To provide these six connections, but minimize the overallwidth of opening 23, shield 18 is connected to the diffusion which formsthe sources of the cells. (The sources of the four cells are commonsince they are a single diffusion.) Furthermore, one of the drainregions of the four shielded cells is connected to cap 19. Consequently,while six connections are required only four polysilicon conductors passthrough opening 23. Five polysilicon conductors pass under opening 22,but one is connected to cap 19. Therefore, the widths of openings 22 and23 are minimized. However, to achieve this, there must be a spacebetween the shield 18 (which is tied to the common source regions) andthe cap 19 (which is tied to a drain region). Radiation entering narrowopening 22 is reflected by the substrate to the metal and manyreflections occur before the radiation reaches the cell area 20. Suchradiation is substantially attenuated and does not significantly erasethe shielded EPROM cells.

FIG. 3 is a topological view of the presently preferred embodiment ofthe invention. The dotted lines on the topological view outline thesecond layer of polysilicon lines. In the presently preferredembodiment, each cell includes a floating gate fabricated from a firstlayer of polysilicon. Each cell 26-29 includes a drain region(underneath buried contacts 37-40) which is coupled through buriedcontacts 37-40 to a second layer polysilicon line 53-56 respectively.

FIG. 4 shows a cross-sectional view of cell 26 as a typical prior artfloating gate EPROM cell. Cell 26 is fabricated on substrate 63 andconsists of a source region 34, the drain region 31, the floating gate32 fabricated from the first polysilicon layer, and the secondpolysilicon gate 33 which is the control gate. Floating gate 32 can beelectrically programmed and erased when exposed to radiation. A similarstructure exists for cells 27-29. The second polysilicon gates of cells26-29 are connected together with polysilicon line 33.

Returning to FIG. 3, the sources 57-60 of cells 26-29 are connectedtogether by the common diffusion 34. The areas 61 marked with an "X" onFIG. 3 indicate the areas in which metal extends into the substrate andinto the diffusion 34. Consequently, the sources 57-50 of cells 26-29are connected to the metal shield 18 via diffusion 34. In a similarmanner, diffusion 35 extends about cap 19. Diffusion 35 is connected tometal dap 19 via area 62 which extends into the substrate and intodiffusion 35. Buried contact 36 connects the diffusion 35 to a secondpolysilicon layer line 53 which in turn is connected to the drain regionof cell 26 through buried contact 37. Consequently, the drain region ofcell 26 is connected to the metal cap 19.

In order to select each of cells 26-29 separately, conductors connectedto each terminal of each cell must be available to be connected to othercircuitry in the EPROM. As discussed above, the drain of cell 26 isavailable to be connected to other circuitry in the EPROM since it isconnected to cap 19. The drains of cells 27-29 are connected topolysilicon lines 54-56 through buried contacts 38-40, respectively.Polysilicon lines 53-56 and gate 33 (a total of 5 lines) cross opening22 separately. Lines 54-56 and the gate 33 continue around cap 19 toopening 23.

Metal (typically aluminum) is deposited above the entire structure ofFIG. 3 and is indicated by the unmarked and unshaded areas. The samemetal flows down to the substrate in the areas 61 and 62 indicated withan "X" to form the walls of the shield 18 and cap 19.

Cap 19 caps off opening 22, i.e. it prevents radiation from enteringopening 22 in the plane of the oxide. Any radiation entering opening 22must enter at an angle almost perpendicular to the substrate.Consequently any radiation which is not absorbed by the substrate willbe reflected toward the metal at a very steep angle. Radiation will thenbe reflected by the metal towards the substrate at a steep angle wherepart will be absorbed and part reflected again. Due to repeatedabsorptions and reflections, the radiation will be greatly attentuatedby the time that it reaches the cell area 20. On the other hand,radiation can enter opening 23 in the plane of the oxide and travel tocell area 20 without significant attenuation. Consequently, it isimportant to minimize the total width of the opening. For this reason,the drain of cell 26 was connected via the buried contact 36 to themetal cap 19 so that although five conductors must pass through opening22, only four conductors must pass through opening 23. Furthermore, thesources of cells 26-29 must also be available for connection to othercircuitry. By connecting the sources to the metal shield 18, the sourceregions can easily be connected to other circuitry or grounded withoutthe need for bringing another conductor out through opening 22 or 23.

The protective properties of the shield can be understood by review ofFIG. 2. Radiation travels in a straight line and radiation will nottraverse sharp bends or angles except as reflected around these angles.First, assume that cap 19 was not part of the structure. Radiation couldenter the shield in the plane of the oxide through the opening 22 andpass directly to side 44 of shield 18 without significant attenuation.It is important that side 44 is parallel to opening 22 so that mostradiation will be reflected off side 44 toward opening 22. Side 64prevents radiation entering opening 22 from reaching the cell area 20without being reflected off side 44. Side 21 should be parallel to side49 and perpendicular to both the cell area 20 and side 44. Radiationentering opening 22 must now be reflected in such a manner that theradiation traverses two angles of approximately 90 degrees beforereaching cell area 20. Radiation entering opening 22 in the plane of theoxide would be very strong when it reached side 44 and thereforesignificant amounts could be reflected and still reach the cell area 20.Therefore, cap 19 is placed across the opening 22 to prevent radiationfrom entering in the plane of the silicon dioxide.

With cap 19 in place, radiation can only enter through opening 23 in theplane of the silicon dioxide. Therefore, it was important to minimizethe breadth of this opening. Radiation entering opening 23 will proceedto side 46 of cap 19. Side 46 should be parallel to opening 22 so thatradiation entering opening 23 will be reflected right back out of theopening 23 by side 46. Radiation entering opening 23 would have to makean angle of approximately 90 degrees to reach opening 22. Consequently,very little radiation which has entered in the plane of the silicondioxide will be reflected through opening 22 towards side 44 and thecell area 20. Furthermore, as discussed above, any radiation reachingside 44 would still have to be reflected so as to traverse two moreangles of approximately 90 degrees before reaching cell area 20.

The most common type of radiation which is used to erase EPROMs isultraviolet radiation. When ultraviolet radiation strikes a siliconsubstrate, approximately 35% of the radiation is absorbed and 65% isreflected. Metal will absorb very little radiation and consequently willreflect almost all radiation striking it. FIG. 5 shows how radiationentering opening 22 would be reflected as it travels towards metal side44. It is known that the refractive index of air is 1 and that therefractive index of silicon dioxide is 2. According to Snell's law,radiation entering from the air into the silicon dioxide will bereflected toward the perpendicular. Light will pass directly through thesilicon dioxide to the substrate where approximately 35% will beabsorbed and 65% will be reflected towards the metal. The metal thenwill reflect almost all of the radiation back to the substrate where itwill be continually absorbed and reflected.

Many variations of the preferred embodiment can be made depending on theamount of radiation which the EPROM cell can withstand. For example, insome circumstances, the cap 19 may not be necessary. In othercircumstances, cap 19 could be connected to shield 18 so that opening 22does not exist and five conductors would pass through opening 23.Additional right angles could be added to the shielding structure sothat radiation would have to traverse more right angles before reachingthe EPROM cell. It is also possible that in certain cases polysilicon orcertain insulators may be sufficiently opaque to be used to form theshield and/or cap. A refresh circuit could be added to reinforce thedata in the shielded EPROM cell. Each time the EPROM cell is programmed,the shielded EPROM cells would be sensed and the data sensed rewritteninto the shielded EPROM cells. Such a refresh circuit would assist inpreventing total erasure of the shielded EPROM cells even if someradiation reached the shielded EPROM cells during the erasure of theEPROM. It will be obvious to one having ordinary skill in the art thatnumerous other modifications and departures may be made in the layout ofthe shielding structure depending on the degree of protection that theshielded area requires, the number of cells to be shielded, and thesubstrate area which is used for the shield; thus the invention is to beconstrued as being limited only by the spirit and the scope of theappended claims.

I claim:
 1. In a metal-oxide-semiconductor (MOS) electricallyprogrammable read only-memory (EPROM) which is fabricated on a substrateand which is erased when exposed to radiation, an improvementcomprising:redundant elements to be used in place of defective elements;a programming means for programming said redundant elements so as tocause said redundant elements to be used in place of said defectiveelements, said programming means including at least one EPROM cell whichis erased when exposed to radiation; and a shielding member comprisingan upper cover and upstanding members extending from said upper cover tosaid substrate disposed about said EPROM cell which causes radiationincident on said memory to be attenuated before striking said EPROM cellthus inhibiting said EPROM cell from being erased when said memory isexposed to radiation; whereby said EPROM cell can be permanentlyprogrammed so that said redundant elements are always used in place ofsaid defective elements.
 2. The memory of claim 1 wherein said shieldingmember is disposed over the surface of said EPROM cell and extendsnormally into said EPROM cell at the edges of said EPROM cell so as tomake contact with said substrate.
 3. The memory of claim 2 wherein saidshielding member is a conductor which defines an opening above thesubstrate so as to allow conductors connected to said EPROM cell to passthrough said opening, said opening formed such that radiation enteringsaid opening must traverse at least one angle of approximately 90degrees before reaching source and drain regions of said cell.
 4. Thememory of claim 3 wherein said shielding member is connected to adiffusion region formed in said substrate, said diffusion being commonwith said EPROM cell so as to reduce the number of said conductorspassing through said opening.
 5. The memory of claim 4 wherein saidshielding member is disposed about a plurality of said EPROM cells. 6.The memory of claim 5 wherein said shielding member is comprised ofmetal.